Dopant concentration dependence of electromechanical coupling coefficients of co-doped AlN thin films for BAW devices

In this paper, we present piezoelectric co-doped (Mg,Hf)
_x
Al
_1-x
N thin films, where the dopant concentration x is in the range of 0-0.13. The (Mg,Hf)
_x
Al
_1-x
N thin films were prepared on Si (100) substrates with a dual magnetron with AC power applied between two ring targets. The compositions of the targets were pure Al and Al-Mg-Hf metals. The composition of the films was controlled by adjusting a DC bias power between the two ring targets. The crystal structures and piezoelectric coefficients d
₃₃
of the films were investigated by X-ray diffraction (XRD) and with a piezometer. XRD measurement showed that the increases of concentration x did not affect the c-axis orientation of AlN and induced a decrease of lattice constant ratio c/a. The d
₃₃
of the (Mg,Hf)
_x
Al
_1-x
N thin films increases as the concentration x increases. The d
₃₃
of the (Mg,Hf)
_x
Al
_1-x
N and our previously reported (Mg,Zr)
_x
Al
_1-x
N have the same linear relationship in respect to the change of the c/a. Furthermore, the film bulk acoustic resonators (FBARs) using these films were fabricated and characterized as a function of the concentration x. The electromechanical coupling coefficients k
²
of fabricated FBARs increased in proportion to the concentration x. These improved k
²
are the direct consequence of the increased piezoelectric constant e
₃₃
in conjunction with the decreased elastic constant C
₃₃
. These behaviors of the (Mg,Hf)
_x
Al
_1-x
N are consistent with those of Sc
_x
Al
_1-x
N, indicating that the Mg-Hf co-doping and Sc doping have the same effect as improving of the piezoelectric properties and k
²
of AlN.